Hi Ronald,
As far as i understood your question you wanted to know why the JCO connection,JCO entry is necessary & why the entry for gateway and application server.
to answear your first question-
example- if user want to view payslip in EP there is an iview for that wherein the JCO connection name to your backend ECC system is mentioned in iview object details so if that JCO connection is not working he wont be able to view his salary when he clicks on the iview.
In summery-The JCO connection details contain the details of the system to which you need to connect.
and regarding the two different entry for message server and application server-
When you try to create a JCO connection for the first time it will ask if you would like to create the connection for load balancing or without load balancing.Accordingly if you choose with load balancing the Message server details entry becomes important and application server entry doesnot hold any relevance & vice versa for connection without load balancing.
Thanks,
Rajdeep